Coating agents need to enable the surfaces of a variety of substrates to have a good design and to enable highly durable coating films to be formed to preclude degradation of the surfaces of substrates. Especially in recent years, there has been an industrial need for a coating agent which enables formation of a coating film having a water resistance as well as excellent solvent resistance which enables substrates to be prevented from being degraded, for instance, by application of detergents or by use of a variety of marker pens.
A coating agent having such characteristics is employed as a coating agent used for protecting the surfaces of metal substrates which easily cause rust and corrosion due to contact thereof with, for instance, water in many cases. Such a coating agent used for protecting the surfaces of metal substrates needs to have a water resistance and high solvent resistance which enable the surfaces of the metal substrates to be prevented from corroding. In particular, in the iron and steel industry in which the surfaces of coating films formed on the surfaces of metal substrates are frequently washed with, for example, an alkaline detergent, the solvent resistance is an important characteristic in terms of prevention of, for instance, removal and dissolution of the coating film and degradation of the metal substrates due to effects of the detergent.
An example of known coating agents which enable formation of a coating film having a good water resistance and solvent resistance as described above is a coating agent that is an aqueous resin dispersion which contains 99 to 60 parts by mass of a polyester resin having a weight average molecular weight of not less than 6,000 and an acid value of 8 to 80 mgKOH/g and 1 to 40 parts by mass of an epoxy resin exhibiting a solubility of not more than 5 mass % in water at 25° C. and in which the maximum dispersed particle size of the resin components are not more than 1.0 μm (e.g., see Patent Literature 1).
Such a coating agent enables formation of a coating film having a water resistance and solvent resistance that are acceptable to some extent. Coating films formed of such a coating agent, however, have not completely satisfied the quality demand, for example, in the field of the above-mentioned surface treatment of metal.
Resin compositions used for coating agents or other materials are generally transported or stored, for example, under a variety of temperature conditions; the term of transporting or storing the resin compositions may be for a maximum of approximately six months in some cases.
In the case where the above-mentioned coating agent is stored under a relatively high-temperature environment for approximately a month, the coating agent readily undergoes gelation and coagulation. Hence, such a coating agent does not have a sufficient long-term storage stability and thus causes a problem such as defective film formation in a process for forming a coating film in some cases.